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204                        METALLURGY OF IRON AND STEEL.
the sulphur was unaccounted for, having probably been carried away in the waste gases. The fact that both sulphur and phosphorus thus escape, in an intangible form and in uncertain quantities, renders quantitative work on basic slags very •unsatisfactory. Moreover,-.a,sample of slag is not always representative, for on some heats portions of the basic additions remain sticking to the hearth, while"on others old accumulations of such deposits dissolve in a charge to which they do not belong.
SEC. XII.—Removal of the slag after melting.—When-the stock is properly charged, the greater part of the basic addition becomes an active agent during the melting of the charge. Especially when ore is used the intense action oxidizes a considerable proportion of the phosphorus during the melting, and the slag, after fusion, contains oftentimes a high percentage of phosphoric acid. The idea has-occurred to numberless metallurgists that this first slag should, be removed, in order to get rid of its phosphorus and silica, and thus give the opportunity for a new and purer slag having a greater dephosphorizing power. There are certain practical difficulties in the way, for the height of the metal in the hearth is always varying with the filling of the bottom and with the frothing of the charge, so that there is danger of losing metal if a tap-hole is opened much below the level of the upper surface of the slag; on the.contrary, if the slag is tapped from its upper surface there is no force to the stream, and it is constantly chilling as it runs. In spite of these troubles, the partial removal of the slag is not uncommon. Complete removal can be accomplished by the use of a tilting furnace, for the entire charge can be poured' out and only the metal returned to the hearth.                                                              •
SEC. Xlm.—Automatic formation of a slag of a- given composition.—Afier removing a large proportion of slag from a heat, it might appear to be difficult to again construct a cirider of just the right composition, but/the records in Tables XI-H and XI-I-show that/such is not the easel for, in the heats there given, a part of the 'slag was removed soon after melting. Quite a difference 'will .be found between the first and second slags,,-butJthe first slag was purposely made high in silica, in .order to save lime. "When it is required to maintain a similar composition throughout the heat, it can be done in basic as well as acid practice, as shown in Table XI-L. Four-fifths of the lime was added with the..charge,.and the,